This study will evaluate the effects of certain genes (MDR-1, CYP3A4, and CYP3A5) on metabolism of the drug sirolimus, an immune-suppressing drug given to transplant recipients to prevent organ rejection. Individual differences in metabolism and excretion of sirolimus affect the patient's response to treatment. Patients who have undergone kidney transplantation at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Transplant Branch and have received sirolimus treatment will be enrolled in this study. DNA (genetic material) will be extracted from blood samples collected from transplant recipients to determine their MDR-1, CYP3A4, and CYP3A5 genotypes. Patient demographic information and data on sirolimus metabolism and excretion will be collected from the medical information system, NIDDK transplant database, and the patients' medical records. The data will be compared among patients with different genotypes (genetic constitution of an individual) and haplotypes (set of genes that code for different proteins but are inherited as a unit) to determine the effect of these gene variations on sirolimus metabolism. Information from this study may be applied to developing better dosing strategies, and thus, treatment outcomes for transplant patients receiving sirolimus.